1. Field of the Invention
The invention relates to the field of radiation therapy systems. One embodiment includes an alignment system with an external measurement system and local feedback to improve accuracy of patient registration and positioning and to compensate for misalignment caused by factors such as mechanical movement tolerances and non-strictly rigid structures.
2. Description of the Related Art
Radiation therapy systems are known and used to provide treatment to patients suffering a wide variety of conditions. Radiation therapy is typically used to kill or inhibit the growth of undesired tissue, such as cancerous tissue. A determined quantity of high-energy electromagnetic radiation and/or high-energy particles is directed into the undesired tissue with the goal of damaging the undesired tissue while reducing unintentional damage to desired or healthy tissue through which the radiation passes on its path to the undesired tissue.
Proton therapy has emerged as a particularly efficacious treatment for a variety of conditions. In proton therapy, positively charged proton subatomic particles are accelerated, collimated into a tightly focused beam, and directed towards a designated target region within the patient. Protons exhibit less lateral dispersion upon impact with patient tissue than electromagnetic radiation or low mass electron charged particles and can thus be more precisely aimed and delivered along a beam axis. Also, upon impact with patient tissue, the accelerated protons pass through the proximal tissue with relatively low energy transfer and then exhibit a characteristic Bragg peak wherein a significant portion of the kinetic energy of the accelerated mass is deposited within a relatively narrow penetration depth range within the patient. This offers the significant advantage of reducing delivery of energy from the accelerated proton particles to healthy tissue interposed between the target region and the delivery nozzle of a proton therapy machine as well as to “downrange” tissue lying beyond the designated target region. Depending on the indications for a particular patient and their condition, delivery of the therapeutic proton beam may preferably take place from a plurality of directions in multiple treatment fractions to achieve a total dose delivered to the target region while reducing collateral exposure of interposed desired/healthy tissue.
Thus, a radiation therapy system, such as a proton beam therapy system, typically has provision for positioning and aligning a patient with respect to a proton beam in multiple orientations. In order to determine a preferred aiming point for the proton beam within the patient, the typical procedure has been to perform a computed tomography (CT) scan in an initial planning or prescription stage from which multiple digitally reconstructed radiographs (DRRs) can be determined. The DRRs synthetically represent the three dimensional data representative of the internal physiological structure of the patient obtained from the CT scan in two dimensional views considered from multiple orientations and thus can function as a target image of the tissue to be irradiated. A desired target isocenter corresponding to the tissue to which therapy is to be provided is designated. The spatial location of the target isocenter can be referenced with respect to physiological structure of the patient (monuments) as indicated in the target image.
Upon subsequent setup for delivery of the radiation therapy, a radiographic image is taken of the patient, such as a known x-ray image, and this radiographic image is compared or registered with the target image with respect to the designated target isocenter. The patient's position is adjusted to, as closely as possible or within a given tolerance, align the target isocenter in a desired pose with respect to the radiation beam as indicated by the physician's prescription. The desired pose is frequently chosen as that of the initial planning or prescription scan.
In order to reduce misalignment of the radiation beam with respect to the desired target isocenter to achieve the desired therapeutic benefit and reduce undesired irradiation of other tissue, it will be appreciated that accuracy of placement of the patient with respect to the beam nozzle is important to achieve these goals. In particular, the target isocenter is to be positioned translationally to coincide with the delivered beam axis as well as in the correct angular position to place the patient in the desired pose in a rotational aspect. In particular, as the spatial location of the Bragg peak is dependent both upon the energy of the delivered proton beam as well as the depth and constitution of tissue through which the beam passes, it will be appreciated that a rotation of the patient about the target isocenter even though translationally aligned can present a varying depth and constituency of tissue between the initial impact point and the target isocenter located within the patient's body, thus varying the penetration depth.
A further difficulty with registration and positioning is that a radiation therapy regimen typically is implemented via a plurality of separate treatment sessions administered over a period of time, such as daily treatments administered over a several week period. Thus, the alignment of the patient and the target isocenter as well as positioning of the patient in the desired pose with respect to the beam is typically repeatedly determined and executed multiple times over a period of days or weeks.
There are several difficulties with accurately performing this patient positioning with respect to the radiation treatment apparatus. As previously mentioned, patient registration is performed by obtaining radiographic images of the patient at a current treatment session at the radiation therapy delivery site and comparing this obtained image with the previously obtained DRR or target image which is used to indicate the particular treatment prescription for the patient. As the patient will have removed and repositioned themselves within the radiation therapy apparatus, the exact position and pose of a patient will not be exactly repeated from treatment session to treatment session nor to the exact position and pose with which the target image was generated, e.g., the orientation from which the original CT scan generated the DRRs. Thus, each treatment session/fraction typically involves precisely matching a subsequently obtained radiographic image with an appropriate corresponding DRR to facilitate the determination of a corrective translational and/or rotational vector to position the patient in the desired location and pose.
In addition to the measurement and computational difficulties presented by such an operation, is the desire for speed in execution as well as accuracy. In particular, a radiation therapy apparatus is an expensive piece of medical equipment to construct and maintain both because of the materials and equipment needed in construction and the indication for relatively highly trained personnel to operate and maintain the apparatus. In addition, radiation therapy, such as proton therapy, is increasingly being found an effective treatment for a variety of patient conditions and thus it is desirable to increase patient throughput both to expand the availability of this beneficial treatment to more patients in need of the same as well as reducing the end costs to the patients or insurance companies paying for the treatment and increase the profitability for the therapy delivery providers. As the actual delivery of the radiation dose, once the patient is properly positioned, is a relatively quick process, any additional latency in patient ingress and egress from the therapy apparatus, imaging, and patient positioning and registration detracts from the overall patient throughput and thus the availability, costs, and profitability of the system.
A further difficulty with accurately positioning the patient and the corresponding target isocenter in the desired position and pose with respect to the beam nozzle are the multiple and additive uncertainties in the exact position and relative angle of the various components of a radiation therapy system. For example, the beam nozzle can be fitted to a relatively rigid gantry structure to allow the beam nozzle to revolve about a gantry center to facilitate presentation of the radiation beam from a variety of angles with respect to the patient without requiring uncomfortable or inconvenient positioning of the patient themselves. However, as the gantry structure is relatively large (on the order of several meters), massive, and made out of non-strictly rigid materials, there is inevitably some degree of structural flex/distortion and non-repeatable mechanical tolerance as the nozzle revolves about the gantry. Further, the nozzle may be configured as an elongate distributed mass that is also not strictly rigid such that the distal emissions end of the nozzle can flex to some degree, for example as the nozzle moves from an overhead vertical position to a horizontal, sideways presentation of the beam. Accurate identification of the precise nozzle position can also be complicated by a cork screwing with the gantry.
Similarly, the patient may be placed on a supportive pod or table and it may be connected to a patient positioning apparatus, both of which are subject to some degree of mechanical flex under gravity load, as well as mechanical tolerances at moving joints that are not necessarily consistent throughout the range of possible patient postures. While it is possible to estimate and measure certain of these variations, as they are typically variable and non-repeatable, it remains a significant challenge to repeatedly position a patient consistently over multiple treatment sessions in both location and pose to tight accuracy limits, such as to millimeter or less accuracy on a predictive basis. Thus, the known way to address gantry and patient table misalignment is to re-register the patient before treatment. This is undesirable as the patient is exposed to additional x-ray radiation for the imaging and overall patient throughput is reduced by the added latency of the re-registration.
From the foregoing it will be understood that there is a need for increasing the accuracy and speed of the patient registration process. There is also a need for reducing iteratively imaging and reorienting the patient to achieve a desired pose. There is also a need for a system that accounts for variable and unpredictable position errors to increase the accuracy of patient registration and alignment with a radiation therapy delivery system.